Violet Club: Quite Possibly the Worst Nuclear Bomb Ever Fielded

The warhead, or physics package, of the Violet Club bomb

When taking a look at the development of British nuclear weapons following the Second World War, it has to be viewed in the context of a piece of legislation in the United States that was passed in 1946- the McMahon Act or the Atomic Energy Act. Sponsored by Senator Brien McMahon of Connecticut who chaired the Senate Special Committee on Atomic Energy, this legislation is better known for its creation of the Atomic Energy Commission and the placement of nuclear weapons development and nuclear applications under civilian rather than military control. However, one consequence of the McMahon Act was the stipulation that nuclear weapons development be restricted from US allies- this affect the United Kingdom and Canada who had provided scientists and support to the wartime Manhattan Project. As a result of being shut out of American nuclear weapons development, the British set about to create their own air-dropped weapon which would be fielded in 1953 at RAF Witttering- though somewhat amusingly the first aircraft that could carry the bomb, designated Blue Danube, the Vickers Valiant, didn't become operational until a year later. The purpose of this wasn't just a message to the Soviets, but also to the United States that Britain was more than capable of fielding its own nuclear deterrent despite the McMahon Act. 

On 1 November 1952 the United States detonated its first fusion bomb (H-bomb) in the Ivy Mike test at Eniwetok Atoll in the Pacific. Given that the British were still shut out of US nuclear development by the McMahon Act, despite the fact that the Blue Danube fission bomb (A-bomb) was still a year out from being operational, strategic imperatives meant that Britain had to develop it's own H-bomb and the program was launched in 1954. In the UK, many military systems were assigned code names under the Ministry of Supply's "rainbow codes"- hence, "Blue Danube". In the development of an H-bomb, the casing had its own code name and the actual warhead, called the physics package, had another code name. The casing of the H-bomb was based on the Blue Danube casing and was designated Violet Club while the physics package was designated Green Grass. 

But before the code names had been settled upon, the British H-bomb had a different name- "Interim Megaton Weapon"- implying that it was a high-yield weapon but not a true thermonuclear or H-bomb/fusion weapon. And this is really at the heart of the history of the Violet Club and its historical legacy. First, it indicates that Violet Club was intended to be a temporary weapon and secondly, it wasn't a fusion bomb as was commonly believed by *both* the Soviet Union and the United States. 

The warhead or physics package of the bomb was based on earlier warhead designs that were named Orange Herald and Green Bamboo. Orange Herald was a lighter version of Green Bamboo and the designs were projected to be the new fusion warheads for the Royal Air Force's V-force, the Blue Steel stand-off missile, and the planned Blue Streak intermediate-range ballistic missile. Testing of Orange Herald showed that it had failed to boost the fission reaction to create a fusion reaction. The failure of the warhead designs left the British scrambling for a high-yield weapon and this became the Green Grass warhead of the Interim Megaton Weapon that was based on design elements of the earlier Green Bamboo and Orange Herald designs. As was the case in the United States, interservice rivalries in Great Britain meant that the Army wanted highly enriched uranium (HEU) for nuclear landmines in Europe and the Royal Navy wanted HEU for the reactors for its planned nuclear-powered submarine fleet. The Royal Air Force was of the feeling that the HEU that had so far been produced in British reactors had to be used or it would be lost to rival services, so that was one of several motivations to rush the Interim Megaton Weapon into service as it would use a significant amount of HEU.

Schematic of the Green Grass warhead showing how the ball bearings were used.

It was the design of the Green Grass warhead that went into the Violet Club that made it for all practical purposes a useless weapon. A hollow sphere of HEU was surrounded by a system of 72 explosive lenses that compressed the HEU to critical mass and detonation. But here was the problem. In the Green Grass warhead, the mass of HEU was in *excess* of the critical mass once compressed by the explosive lenses. That meant if the warhead were crushed or damaged during handling, it could partially detonate. American designs avoided this by having an HEU core that was inserted into the physics package usually by the bombardier once the bomber was in flight, thereby "arming" the bomb once the core was inserted. Without the core inserted, the HEU mass in the American designs was below the critical mass. The solution by British designers was to fill the center of the HEU sphere with 20,000 steel ball bearings to prevent the sphere from being crushed and reaching critical mass. To arm the bomb, a plastic plug was removed from the bottom of the warhead (accessible via a hatch on the underside of the Violet Club casing) that allowed the bearings to flow out, thereby arming the bomb. 

While it may sound like a creative solution, there were several issues: 

• The weight of the ball-bearings increased the bomb's weight to 11,250 lbs which was greater than the capacity of not only the bomb release mechanisms of the V-bombers but also the ground-transport equipment of the bomb. 
• The outflow of bearings took at least half an hour under ideal conditions- in cold weather, the bearings could freeze together, making arming the weapon near-impossible. 
• Once the bomb was armed by allowing the ball-bearings to flow out of the center of the warhead, there was on way of making the weapon safe again. In fact, engine running was prohibited even with Violet Club "safed" as it was feared vibration would cause the plastic plug to fall out and inadvertantly arm the weapon. 
• Because the bomb was armed irreversibly, airborne alerts were impossible because take off and landing were too hazardous to attempt with an armed Violet Club. 
• Dispersal of the V-force to outlying fields was impossible as the bomb couldn't be flown to the dispersal airfield and the bomb transport equipment couldn't handle the Violet Club when it had its ball-bearings in place. 

The Blue Danube- the Violet Club looked similar externally.

While the Air Staff of the RAF ordered twelve Violet Club bombs, only five were made and as British author Chris Gibson put it in his book Vulcan's Hammer "From the RAF's point of view, that was five too many." With such an unwieldly weapon, why was it even fielded? First of all, remember that the British were classifying the Violet Club as megaton-class weapon by calling it the Interim Megaton Weapon. It definitely wasn't a megaton weapon, perhaps more 400 kilotons at best, but certainly the Operation Grapple tests at Christmas Island in 1957 did indicate to the Americans the British were succeeding at fielding their own H-bombs- even if those test detonations failed to created the desired thermonuclear reaction. So who was the target of the Violet Club? While serving notice to the Soviet Union that Britain was still a force to be reckoned with, it seems that perhaps the Americans were the target, so to speak- with a weapon in their inventory called Interim Megaton Weapon implying that newer designs forthcoming and the Grapple series of tests in 1957 making a good show of things despite failing to work as planned, in 1958 the United States repealed the McMahon Act and resumed full nuclear cooperation with the United Kingdom. The Green Grass warhead used in Violet Club would be the last all-British nuclear weapon as a new Mutual Defense Agreement signed as part of the repeal of the McMahon Act meant British designers now had access to more advanced and compact American designs. In fact, the successor to the much-despised Violet Club, the Yellow Sun Mk.2, used an Anglicized American Mk.28 thermonuclear warhead. But no other fission weapon ever fielded by any other nation approached the explosive yield of the Violet Club.

Source: Vulcan's Hammer: V-Force Projects and Weapons Since 1945 by Chris Gibson. Hikoki Publications, 2011, p47-51. http://www.nuclear-weapons.info/vw.htm, by Brian Burnell.

Operational Improvisation: Over-the-Shoulder Nuclear Bombing

F-84Gs of the 20th FBW had lightning markings, each squadron had its own color

In 1952 the Republic F-84Gs of the 20th Fighter-Bomber Wing crossed the Atlantic supported by aerial refueling to set up shop at their new base, RAF Wethersfield, in order to provide tactical nuclear strike capability for the first time to NATO forces in Europe. Just a year earlier, scientists and engineers at Sandia, one of the development centers in the United States for nuclear weapons, had developed the Mark 7 nuclear bomb, the first tactical nuclear weapon with an explosive yield of 20 kilotons. While the Mark 7 weapon would be carried operationally by many USAF and US Navy tactical attack aircraft, the first aircraft to carry the Mark 7 operationally also happened to be the first production tactical fighter to have not just nuclear capability, but also air-refueling capability. That was specifically the G variant of the Republic F-84 Thunderjet, which had an air refueling receptacle for a flying boom in the left wing root, a more powerful jet engine, and provisions for the Mark 7's special pylon that had the necessary circuitry for nuclear weapons delivery. 

The Mark 7 was the first American tactical nuclear weapon

The 20th FBW had been given six months to prepare for the move to Great Britain as well as to become the first tactical nuclear fighter-bomber unit in military history. At the time of the deployment, the F-84Gs and pilots of the 20th FBW were only versed in clear-weather weapons delivery more suited to the bombing ranges in the predominantly sunny southwestern United States, drops being made starting at 20,000 feet in altitude. The weather in Europe, however, was far from ideal for this sort of weapons delivery mode, with a predominantly cloudy maritime climate in the areas that the 20th FBW was expected to operate. I had posted this past September about the nuclear delivery role assigned to the McDonnell F-101 Voodoos of the 81st Tactical Fighter Wing based in RAF Bentwaters/Woodbridge in the late-1950s and early 1960s. Not unlike the conditions facing the pilots several years later that flew the Voodoo, the pilots of the 20th FBW were expected to navigate visually and by dead reckoning to their targets with only the most basic of navigational aids. By the time the 20th FBW had set up shop at RAF Wethersfield, a different form of nuclear delivery was needed and the wing commander, Colonel John Dunning, had sent some of his best pilots to Kirtland AFB in Albuquerque, New Mexico near Sandia, to find out more about a new weapons delivery tactic called LABS- Low Altitude Bombing System. Developed at Kirtland by Major Jack Ryan, it hadn't yet received much attention as most nuclear delivery tactics of the day concerned use by strategic bombers. Col. Dunning wanted his pilots in Europe to have every advantage possible and LABS offered that prospect. 

In a LABS run, an IP (initial point) is chosen that is a known distance and direction from the target and was most ideally located three miles away. The LABS equipment was quite basic- it was a timer with a gyro that was free to move about the pitch axis of the delivery aircraft. Having previously set the time from the IP to the pull up point near the target and the calculated angle of release beforehand, an aircraft on a LABS run headed towards the target at 500 mph at low level with the IP in between the aircraft and the target. Once the IP was reached. the pilot pushed the "pickle" button on the control stick which started the timer and a flashing red light on the gunsight was connected to both the timer and the LABS gyro. The pilot pulled into a steady 4G climb and at a precise point and angle (usually 25 to 30 degrees), LABS released the nuclear weapon which continued onward on a precalculated ballistic trajectory towards the target while the pilot pulled his aircraft into an Immelman loop and exited the area as fast as possible. In effect, LABS "tossed" the bomb towards the target. 

Diagram showing the over-the-shoulder bomb toss

On return to Europe, the pilots that trained in the LABS technique for the 20th FBW found that finding an IP near the target was challenging. It was noted that as the distance between the IP and target decreased, the ideal release angle of the Mark 7 bomb increased. If the IP was very close to the target itself, then the optimum release angle was 90 degrees. Pushing the idea further, the pilots of the 20th FBW worked out that if the IP was the target itself, then the optimum LABS release angle was 110 degrees and the bomb would impact right at the point where the pull-up maneuver was initiated. No IP was needed- the target itself was the IP. The bomb was released "over the shoulder" and would arc upward to 10,000 feet and more than a minute elapsed before it detonated, allowing time for the F-84G to rollout and accelerate out of the area in a dive. The USAF and the specialists at Kirtland AFB doubted if the average USAF pilot could carry out such a complex maneuver as the wings had to be absolutely level in the pull up or the bomb's impact point would stray away from the target. The operations officer of one of the 20th FBW's constituent squadrons had noted that the F-84G didn't even need a LABS gyro- the aircraft's own gyro started to "tumble" right past vertical and by complete coincidence, right at 110 degrees! Major John J, Kropenick, the ops officer who made this observation, came up with his own solution, the "Kropenick Autopilot" that was taught to all the pilots of the 20th FBW- two large rubber bands were hooked to the control stick on the run in, each one then looped over a cockpit light on the sidewall on each side. The tension of the rubber bands kept the stick precisely centered during the pull up and once the Thunderjet's own gyro tumbled, the bomb would be released. Pilots taught the method with the "Kropenick Autopilot" had bomb scores acceptable to the USAF given the 20-kiloton yield of the Mark 7 bomb.

By the time the LABS equipment had been fitted to the 20th FBW's Thunderjets, the pilots of the wing had gotten quite proficient at using the "Kropenick Autopilot" and made the transition to using the LABS equipment for "over the shoulder" toss bombing with a minimum of delay and fuss. 

Source: Aviation History, March 2011, Volume 21, Number 4. "Over-the-Shoulder A-Bombing; Cold War F-84G pilots improvised a surprising twist on bomb delivery" by David Rust, p54-57.

If there was any one factor during the Cuban Missile Crisis of 1962 that could have escalated the situation out of the control of President Kennedy and Premier Khrushchev, it wasn't the threat of an American invasion of Cuba and believe it or not it wasn't the presence of the SS-4 and SS-4 medium-range ballistic missiles that targeted most of the Continental United States. It was the presence of tactical nuclear weapons not just on Cuba itself (2 kiloton warheads on the battlefield FROG rockets) but the tactical nuclear weapons on the periphery of the crisis that had release authority vested in lower-ranking officers.

Case in point, each of the five diesel-electric submarines the Soviet Navy deployed to Cuba to monitor the merchant fleet that was bringing war materiel and personnel the island each had single nuclear-tipped torpedo amongst the conventionally-armed torpedoes normally carried. There were tense moments and the quarantine line Kennedy ordered ships carrying military cargoes bound for Cuba had to turn around or stop and be inspected by US Navy warships and in some cases, US ASW forces were tracking and hounding these very submarines, tempting their captains to fire their single nuclear torpedo. But it didn't happen there, thankfully, but there was a routinely scheduled U-2 flight from Alaska half a world away that nearly triggered an escalation of the Cuban Missile Crisis.

On the same day on 27 October 1962 Major Rudolf Anderson, Jr's U-2 flight was shot down over Cuba by an SA-2 SAM missile, a world away at Eielson AFB in Fairbanks Major Chuck Maultsby took off in a U-2 headed to the North Pole on a high-altitude sampling flight to try and collect particle debris from Soviet nuclear tests on the Arctic island of Novaya Zemlya. Upon collecting the required samples, Maultsby set course for Alaska but erred in his navigation and instead started heading towards the eastern peninsulas of Siberia. A rescue Douglas C-54 Skymaster airborne to support his flight noted the start of sunrise and Maultsby reported he didn't see a sunrise indicating he was far to the west of the return path and entering Soviet air space.

US radar sites in Western Alaska detected the MiG-19 fighter scrambled to intecept the lost U-2 who made an immediate turn to the east post-haste! At the same time, two USAF Air Defense Command Convair F-102 Delta Dagger interceptors were scrambled to counter the MiGs and guild the U-2 home. On each Delta Dagger were nuclear-tipped GAR-11 Nuclear Falcon missiles.

Five days earlier when the Strategic Air Command and the US armed forced worldwide went to DEFCON-2 (one stage of full war with the Soviet Union), tactical nuclear weapons like the GAR-11 Nuclear Falcon were loaded on ADC interceptors and entrusted to lower-ranking officers. The GAR-11's warhead was only 0.25 kiloton, but there were larger warheads equally entrusted like the 40-kiloton warhead used on the Nike-Hercules SAM sites that protected the US industrial centers.

So now over the Bering Sea, nuclear armed F-102 Delta Daggers have scrambled under DEFCON-2 preparations to rescue an errant U-2 under pursuit by two Soviet MiG-19s. Maultsby didn't have enough fuel to land back at Eielson AFB but made a deadstick landing at a small Alaskan airfield and the MiGs broke off pursuit and returned to the bases. The Air Defense Command F-102s never got into missile engagement range of the MiG-19s, but the event sent reverberations through Washington and Moscow. Moscow saw the U-2 flight as a prelude to a nuclear bomber strike over the North Pole and Khrushchev protests vigoruously on the 28th to Kennedy, berating him for this "provocation". Kennedy, in a rare moment during the Cuban Missile Crisis, was forced to apologize to his protagonist and to seek every measure to make sure it doesn't happen again.

Kennedy was pissed, to say the least, berating his special Executive Committee of close advisors (ExComm) expasperated "There's always some sonofabitch that doesn't get the message!" Privately Khrushchev consigned the U-2 flight as a navigational error and not a true prelude to an attack- quite possibly the smartest decision he made during the crisis!

Source: DEFCON-2: Standing on the Brink of Nuclear War During the Cuban Missile Crisis by Norman Palomar and John D. Gresham. John Wiley and Sons, Ltd, 2006, p-120-153.

The CIM-10/IM-99 Bomarc was one of the first very long range surface to air missiles to go into service, becoming operational with NORAD for the air defense of North America in 1959. Housed in a rectangular semi-hardened shelter, on receiving the launch order the shelter's roof would slide open and the Bomarc would be raised into the vertical launch position. An Aerojet General LR59 liquid-fueled booster rocket would accelerate the Bomarc to over Mach 2.5 and 60,000 feet when the twin Marquardt RJ43 ramjets took over to propel the missile to over 200 miles out from launch where its 10-kiloton W40 nuclear warhead would destroy the target bombers.

The booster rocket of the missile used hypergolic fuels- red fuming nitric acid as an oxidant and aniline fuel that would spontaneously ignite when mixed. The fuels were stored on the missile for 90 days at a time. When a launch order was received, a helium tank on the missile would be pressurized to provide propellant tank pressurization for the booster rocket. It would take 15 seconds to pressurize the tank, during which time the Bomarc was raised to the vertical position for launch. At the end of each 90 day period, the missile would have to be defueled, decontaminated and then refueled. Pressurized helium would be used to empty the tanks to defuel them as well as in the refueling procedure.

On 7 June 1960 the firefighting department of Fort Dix and McGuire AFB, New Jersey received an emergency call that a fire had broken out in Bomarc Shelter #204. The missile in that shelter was nuclear tipped and the failure of a helium tank set up a pressure shockwave that ruptured the booster rocket propellant tanks, spilling the hypergolic fuels and starting a fire. The fire then heated the remaining fuel in the missile, causing an explosion. Fortunately the shelter roof doors were closed and took the brunt of the propellant explosion, but the steel roof and blast doors were still blown off the shelter and the fierce fire burned for 45 minutes. The missile and launch equipment were destroyed and the fire was so intense the steel structural beams of the launch shelter sagged from partially melting.

Fortunately the hardened shelter design contained most of the explosion and debris and the adjoining launch shelters were undamaged. Water had to be pumped on the remains of the shelter and missile through the night before it cooled down enough for radiation safety teams from the Atomic Energy Commission and the USAF to survey the damage. As the fire had to burned, the structure of the missile melted and the nuclear warhead fell right into the fire and was itself partially melted. Careful study of the remains of the warhead showed that between 2 to 11 oz of plutonium were lost but the tritium tank that held the heavy hydrogen isotope to trigger the nuclear reaction was intact.

The plutonium caused radiation counts inside the remains of the shelter to soar to 2 million counts per minute as the salvage teams had to wear protective gear. What was left of the floor and shelter were sprayed with a special thick paint that absorbed the radiation (thankfully there was no high energy gamma radiation from detonation of the warhead) and four inches of concrete were poured on the surrounding area until detected radiation counts were zero. The site remained fenced off until the Bomarc missiles were decommissioned in 1972. In 2004 the USAF had the site demolished and had 20 feet of soil surrounding the shelter excavated. The soil and the debris were shipped out to a nuclear waste repository in Utah.

Source: Airpower, September 2004, Volume 34, No. 9. "Nuclear Nightmare Almost" by Dr. Richard V. Porcelli, p13.

The Politics Behind the French Stratotanker Purchase

On 13 February 1960 France exploded its first thermonuclear weapon, a 60-kiloton device in the Algerian desert, making France the fourth member of nuclear-capable powers. In December of that year, President Charles De Gaulle formally announced his intentions to establish an autonomous nuclear strike force independent of US control based on the Dassault Mirage IV supersonic strike bomber which had made its first flight the year prior.

But the Mirage IV lacked the range to reach targets in the Western Soviet Union and defense planners in the Armee de l'Air (AdA- French Air Force) wanted to acquire an aerial tanker that could launch at a moment's notice with the Mirage IV force, refuel the bombers at high speed and high altitude, and offload fuel quickly to the Mirages. Only the Boeing KC-135 Stratotanker met these stringent requirements and negotiations began to purchase 10 KC-135As- 9 tankers for every four Mirage IV in the Force de Frappe (Strike Force) plus one spare. However, with the 1960 election of John F. Kennedy came a change in US nuclear strategic doctrine from massive retaliation to one of flexible response- to the new US president, his Secretary of Defense Robert McNamara, and his Secretary of State, Dean Rusk, an autonomous French nuclear deterrent would undermine the new strategic doctrine and increase the chance of premature use of nuclear weapons without American oversight.

The sale of the KC-135 to France was seen as equal to nuclear proliferation as the tankers were necessary for the Force de Frappe to reach Soviet targets. By 1962 France increased its Mirage IV force to 50 aircraft and wanted more than just 10 tankers which were agreed on principle by military officials in the US Defense Department. President Kennedy, however, vetoed the sale and even announced at a press conference that the tanker sale was a dead issue. On that very same day, however, the Deputy Secretary of Defense Roswell Gilpatrick approved the sale of 12 KC-135s to be designated C-135FR (ostensibly to hide their purpose as tankers for the Force de Frappe).

Two weeks later at a commencement address at the University of Michigan at Ann Arbor, Defense Secretary McNamara reiterated official US policy of opposing any aid that allowed an independent French nuclear deterrent. Despite this, a month later, both McNamara himself and Secretary of State Dean Rusk signed off on the tanker sale worth $50 million. At the time the sale was rationalized as an offset for American gold losses to France and concern by the US Treasury Department about foreign accumulation of gold. However, with the end of the Cold War, declassified documents show the sale was undertaken quietly in an effort to improve strained relations between France and the United States.

The first AdA C-135FRs arrived in France at Istres in 1964 following completion of KC-135 crew training by the French at Castle AFB in California. The last C-135FR arrived in October of that year which coincided with the Force de Frappe's first operational Mirage IV nuclear alert. Initial Mirage IV operations included a 24-hour airborne alert of Mirage IV bombers supported by the C-135FRs, but these were ended in 1967 as they could not be afforded by France unlike the US Strategic Air Command's "Chrome Dome" airborne alert.

In 1966, President De Gaulle went ahead and pulled France out of NATO's military command, an action that was only reversed in March of 2009 when President Nicholas Sarkozy signed a decree supported by the French legislature re-integrating France into NATO.

Source: Boeing KC-135 Stratotanker- More Than Just A Tanker by Robert S. Hopkins III. Aerofax, 1997, p73-75.

During the development of the hydrogen bomb there were concerns by the USAF that the delivery aircraft wouldn't be able to successfully exit the target area after dropping the weapon. As the development of the Atlas ICBM was still years away, Project Brass Ring was initiated in 1950 to create an unmanned version of the Boeing B-47 Stratojet to carry the H-bomb which would be designated MB-47 which in turn would be controlled by a manned Stratojet designated DB-47.

North American Aviation developed a sophisticated navigation and flight control system for the MB-47- but given the Air Force's requirements that the system be fully automatic, jam-resistant, and be accurate, it pushed the technology of the day and by 1952 it had to be cancelled due to cost overruns. The USAF then turned to Sperry Gyroscope to pick up where North American left off and it eventually cost a then-hefty sum of $2.3 million to complete. The MB-47 made its first flight on 7 May 1952 and by that summer both the MB-47 and the DB-47 controller had made several test flights with encouraging results. However, the cost of Project Brass Ring had nearly doubled from $4.9 million to just over $10 million.

By this point in the program SAC had determined that the Convair B-36 Peacemaker would be able to deliver the first production H-bombs if they were equipped with a parachute-retarding lay-down system. Furthermore, gaining permission from NATO members to forward deploy Stratojets in the UK, Spain, and French Morrocco made Project Brass Ring unnecessary and the program was terminated in April 1953.

Source: Boeing's B-47 Stratojet by Alwyn T. Lloyd. Speciality Press, 2005, p206-207

The first atomic bomb to be mass produced by the United States was the Mk-6. Designed for carriage by bombers only, it was intended for use against area targets. With a 40 kiloton warhead, the Mk-6 was carried by the B-29/B-50 Superfortress, B-36 Peacemaker, B-47 Stratojet, B-52 Stratofortress, and the Navy's AJ-1 Savage. Deployed in 1951, it was the first new strategic weapon since the Fat Man that was dropped on Nagasaki during World War II.

The Mk-6 had its nuclear materials in a special capsule that was inserted into the bomb unit by the bombardier before it was dropped. Designed as a safety measure, having the nuclear material in a separate capsule undoubtedly prevented several accidental detonations in the early 1950s.

Being an early fission weapon, the Mk-6 required a lot of nuclear material to produce its desired yield. With the advent of thermonuclear weapons in the late 50s it then became possible to have the same explosive yield with less material and the Mk-6 bombs in the inventory became a valued source of nuclear material for the newer generation of smaller weapons, with the last Mk-6 being withdrawn from service in 1957.

Source: Nuclear Weapons of the United States; An Illustrated History by James N. Gibson. Schiffer Publishing, 1996, p89.

The Republic F-84G was not only the most widely produced version of the F-84, but was also the first single-seat fighter that was nuclear-capable, able to carry and deliver via LABS the Mark 7 nuclear bomb which itself was the first tactical nuclear weapon for US forces. The Mark 7 had a variable yield. So that the F-84G could deliver its weapon deep into enemy territory, it also became the first single-seat fighter to have a slipway refueling receptacle on the upper port wing to take fuel from boom equipped KB-29s or KC-97s.

Source: International Air Power Review, Volume 24. "Warplane Classic: Republic F-84" by David Willis, p137-138.